1. Field of the Invention
The invention relates to the use of a coal combustion product, fly ash, as an additive in concretes. More particularly, the invention provides methods of treating Class C fly ash, and concrete mixtures containing Class C fly ash, to prevent the formation, in sulfate ion-containing environments, of volume-expanding compositions within the hardened concrete that cause stress failure of the concrete.
2. Description of the Related Art
Fly ash is a combustion product produced when coal is burned in power plants. The large quantity of fly ash produced nationally poses a disposal problem. All fly ash, however, is not the same. The chemistry of the fly ash is dependent upon the nature of the coal from which it is obtained. Thus, ASTMC618 (incorporated by reference) defines two classes of fly ash: Classes C and F. Class F is obtained by burning anthracite or bituminous coal while Class C is the combustion product of sub-bituminous coal or lignite. Class C fly ash often contains significant amounts of calcium mineral matter, while Class F rarely does. Thus, Class C fly ash has cementitious and pozzolanic properties while Class F is rarely cementitious when mixed with water alone. "Use of Fly Ash in Concrete," Reported by American Concrete Institute Committee 226 (1987) (hereby fully incorporated by reference).
The Class C fly ashes contain calcium aluminate compounds, calcium hydroxide, and/or iron oxides. Any of these three components may lead to expansion and cracking of hardened concrete made from a Class C fly ash-cement mix when it is exposed to a sulfate environment.
The art, while speaking to certain mixtures of "fly ash" with cements, does not address the problem of concrete failure when a Class C fly ash-containing concrete is exposed to a sulfate ion-containing environment. For example, German Offenlegungsschrift 28 01 687 (German '687) contains an English language abstract which discloses a "binder" that contains a "high percentage of fly ash" that is used to make high strength concrete or concrete moldings. The binder contains 72-82 wt. % fly ash, 16-24 wt. % hydraulic binder, and 2-4 wt. % gypsum. The preferred hydraulic binder is clinker, cement, or hydraulic lime. The mixture may further contain 4-6 wt. % silica sand. In the process, the fly ash, hydraulic binder, and gypsum are milled together to a certain fineness. It is preferred that the fly ash and sand are first milled to a certain fineness and thereafter milled with the cement and gypsum. There is no discussion of the sulfate resistance of the concrete obtained from this "binder" containing fly ash.
Likewise, British patent 940,692 (British '692) discloses that clinker may be simultaneously crushed in its crude or semi-crude state with fly ash. Page 2, lines 32-34. It also discloses first crushing clinker alone and subsequently mixing it with the fly ash in a final stage of crushing the ash and clinker together. Uncrushed ash will not produce an effective cement. Page 2, lines 35-39. Granulated blast furnace slag may be used in certain of the blends. British '692 indicates that in its cements, the proportion of fly ash ranges from 20 to 30 wt. % and the proportion of portland cement or clinker and optimal granulated slag present in the cement is from 70 to 80 wt. %. In the examples, specifically Examples 2 and 11, the compositions include clinker, fly ash, and gypsum in the following proportions:
______________________________________ Example 2 Clinker 25% Fly ash 73% CaSO.sub.4 1% Na.sub.2 SO.sub.4 1% Example 11 Clinker 30 parts Fly ash 70 parts Gypsum 5 parts ______________________________________
Kovach, The Use of Thermal Power Station Pulverized Fuel Ash in the Manufacture of Cement in Hungary, (Ankara, Turkey Symposium, Nov. 1970), indicates that grinding increases the reactivity of fly ash and the strengths of cement containing the ground ash. See pg. 4. However, Kovach also indicates that the additional of fly ash improves the sulfate resistance of the cement. Testing indicated that the strength of cement containing 30% fly ash was from 5 to 20% higher than that of the referenced cement without fly ash. See pg. 8. In certain tests, gypsum was added to the fly ash and cement so as to give a 7% sulfate content in the cement. The expansion of the gypsum-containing fly ash and cement mixtures was less than for the cement-fly ash mixtures without gypsum, when measured at ages 28 days, 90 days, and one year. See pg. 8. Kovach concludes that the expansion of fly ash-containing cements slows down or ceases after some time, while that of control cements increases until the specimens are totally destroyed. In light of this, Kovach concludes that fly ash-containing cements surpass ordinary portland cements with respect to sulfate resistance. However, he found that the addition of gypsum to fly ash-containing cements reduced the volume expansion of specimens.
Davis, et al., Properties of Cements and Concretes Containing Fly Ash, Proceedings of the American Concrete Institute (Feb. 1937),, Vol. 33, Journal of the American Concrete Inst., indicate that fly ash and portland cement may be mixed by intergrinding. See pgs. 580-81, 597-98. Further, Davis suggests that whereas concretes typically undergo shrinkage, fly ash-containing concrete exhibit a "fairly low shrinkage" and shrink less than corresponding concrete without any fly ash. See pgs. 603-04. With respect to resistance to sodium sulfate, Davis indicates that only two fly ash-cement specimens tested were not equal in resistance to corresponding fly ash-free portland cement concretes. There is no indication that the fly ash used was of the Class C type.
U.S. Pat. No. 3,782,985 uses "cenospheres"--small hollow spheres of fly ash--to make light weight concrete by admixing with portland cement. The resultant concretes are not only light weight but also have high strength, although the cenospheres are mixed into cement in the ratio of 0.2 to 4 parts fly ash to 1 part cement, by volume. There is no indication that the concretes lack sulfate resistance.
U.S. Pat. No. 3,565,648 to Mori, et al., shows the grinding of fly ash with gypsum, in a hydraulic slurry. The ground slurry is then added to a cement mixture in a certain proportion to prepare concretes. During the grinding of fly ash with gypsum, in the presence of water, alumina from the fly ash reacts with calcium sulfate and water to form a hydrate of calcium sulfoaluminate. Col. 1, ln. 66-Col. 2, ln. 7. Thus, the process of Mori requires inter-grinding of fly ash with gypsum in the presence of water.
European patent application 007 872 A1 shows a hydraulic dry concrete mixture containing cement, fly ash, and an accelerator. The accelerator is disclosed as being selected from slaked lime, gypsum, or activated silica. The mixture is said to exhibit improved setting characteristics and produce concretes with high mechanical strength. The mixture includes 40-70 parts portland cement, 20-50 parts fly ash, and 0-15 parts of the accelerator.
U.S. Pat. No. 4,240,952 to Hulbert, et al., discloses a concrete composition including gypsum, portland cement, and an aggregate containing fly ash. While noting that it had been proposed to substitute fly ash for certain proportions of portland cement, the '952 patent indicates that portland cement generally contains 2 wt. % of gypsum, added as a retarder. The chemical reactions during setting of concrete results in the formation of four important chemicals: tricalcium aluminate which hydrates rapidly producing heat and causing initial stiffening of the concrete; tricalcium silicate which jellifies within a few hours and has an effect on the strength of concrete mainly in the first 14 days; dicalcium silicate which forms slowly and which is mainly responsible for progressive increase in strength which occurs from 14 to 28 days and beyond; and tetracalcium alumino-ferrite which has no effect on the strength and other properties of the hardened cement.
While it is noted that commercial portland cement normally contains about 2 wt. % of gypsum, in the '952 composition, an additional amount of gypsum is added to act as a retarder by forming a sulfo-aluminate layer around particles of portland cement and fly ash. This layer slows down the dissolution of alumina and lime and thus inhibits hydration. While it is asserted to be within the scope of the invention to replace all of the portland cement with fly ash, the invention includes 20 percent by weight of portland cement in the cementitious material in order to form a mix that resembles portland cement mixes.
U.S. Pat. No. 4,715,896 to Berry discloses a water-hardenable cementitious binder composition that includes finely divided blast furnace slag, Class C fly ash, and an alkaline activator that increases the pH of the composition above about 11 in the presence of water. It is disclosed that the alkaline activator may be any alkaline metal or alkaline earth metal hydroxide, compatible with the other ingredients of the composition, that will elevate the pH of the composition in the presence of water. It is admitted that the nature of the cementitious reaction which takes place between the slag material and fly ash in the presence of the activator is not fully understood. However, it is asserted that elevation of pH is required. See Col. 2, lns 36-54. Significantly, however, the composition of the '896 patent includes blast furnace slag and does not include portland cement.
U.S. Pat. No. 4,756,761, to Philip, et al., discloses cementitious products including fly ash that require the addition of water and an alkaline material, preferably lime, to activate the cementitious composition. It further discloses that a sulfate anion can be added as either gypsum, or sodium sulfate, or another. The patent appears to encourage the formation of ettringite-like structures in order to obtain sufficient strength and stability in the final cementitious product. Col. 4, lns 66-Col. 5, ln. 2.
There yet exists a need for a method for disposing of the large quantities of Class C fly ash that are generated as a product of lignite and sub-bituminous coal combustion at power generating facilities. In order to allow use of this fly ash in concrete, methods must be found to overcome the stress cracking of hardened concretes containing Class C fly ash when they are exposed to a sulfate environment.